A rosemary plant can be useful and healthy without carrying flowers. If the shoots are growing and the foliage looks sound, a missing display is a reason to review its conditions, not immediately increase fertiliser.
Start with light. Compare where the plant sits now with where it stood during active growth. A bright-looking room can still leave a shrub well away from direct daylight. Move it gradually if it needs a brighter position.
Think back to the last trim
Repeatedly shortening every new shoot changes the amount of mature growth available. Keep a record of when you trimmed it and what you removed. Avoid another heavy cut simply because the flowers have not appeared.
Next check the pot. Compost that remains wet for a long time, a blocked drainage hole or a saucer full of water needs attention before feeding does. Deal with the root environment first.
Flowering also varies with the variety and local season. Compare your plant with identified rosemary growing nearby. A flowering date quoted for a different climate is a poor deadline for your own pot.
Related reading: When Does Rosemary Flower? Blooms and Flowering Season.